5x in my Opinion. NFC and faster updates. Camera is supposed to be good too but I'd wait till reviews before you make a concrete decision. The features you may miss out on on the Oneplus can easily be added through roots. Nexus phones are usually easy to root

Hello!
I've been a proud Nexus fanboy for some time. I started with the Nexus 4 and last year I was tempted with the Nexus 6. However.... It looked like BIG phone and an expensive one at that. I started truffling around Oneplus and eventually decided to order a Oneplus One. This meant I had to ingratiate myself with the community, which meant write a big article about why I wouldn't be buying the Nexus 6 and giving succinct reasons why the Oneplus would probably be a better purchase. I got my invite and bought that phone a year ago.
Let me tell you, the Oneplus is a desirable object. It's so affordable and so crammed full of tech. It's fast, the battery longevity is impressive, it's screen is more than adequate, it even has the distinct quality of being completely unknown to the masses. The only problem I consistently found with the handset was ....
IT IS LIKE TRYING TO COMMUNICATE WITH SOMEONE USING TWO TIN CANS AND A PIECE OF STRING.
If that isn't an issue, i'd recommend it every day of the week. Apart from the flaky operating system, I still felt cutting edge. I would forgive the occasional blip, the odd peccadillo. I'd turn a blind ear when I put my hand accidentally over the speaker.
It's a great handset for the price. I strongly suspect the Oneplus 2 will also be great for the price. I even suspect that if you keep the handset relatively clean of Apps you'll find the call quality remains adequate. It's only when the months accrue and the architecture becomes calcified with Added/Removed apps that you feel the handset beginning to creak and groan.
Android isn't removed from this issue. My Nexus 4 had all sorts of issues and quirks. But the call quality always seemed to work.
I would therefore recommend the Nexus 5x. Why? I've just bought the 6p and OMG this is such an amazing piece of work. The 5x is basically the same, but slower and without the aluminium. It's slower because of RAM and processor. It's not Tiny Tim ... the handset is still relevant and quick by all accounts. You'll probably get annoyed by the wobble when typing on a desk because of the camera protuberance .. that's why I love the 6p's flat visor (and also why I love the Google case to cover up that ugly visor).
Go 5x ... it's solid. It's not quirky. We all love quirky ... for one night. The Oneplus 2 is not a steady relationship. If you're looking for marriage, this year go for the 5x or 6p. They're practically perfect in every way [apart from the myriad ways in which handsets differ of course].